The subject matter described herein relates generally to an energy exchange system for conditioning air in an enclosed structure, and more particularly, to an energy exchange system having at least one energy recovery device and a moisture control loop, which may circulate a liquid desiccant, for example.
Enclosed structures, such as occupied buildings, factories and animal barns, generally include a heating, ventilation, and air-conditioning (HVAC) system for conditioning ventilated and/or recirculated air in the structure. The HVAC system includes a supply air flow path and a return and/or exhaust air flow path. The supply air flow path receives air, for example outside or ambient air, re-circulated air, or outside or ambient air mixed with re-circulated air, and channels and distributes the air into the enclosed structure. The air is conditioned by the HVAC system to provide a desired temperature and humidity of supply air discharged into the enclosed structure. The exhaust air flow path discharges air back to the environment outside the structure, or ambient air conditions outside the structure. Without energy recovery, conditioning the supply air typically requires a significant amount of auxiliary energy. This is especially true in environments having extreme outside or ambient air conditions that are much different than the required supply air temperature and humidity. Accordingly, energy exchange or recovery systems are typically used to recover energy from the exhaust air flow path. Energy recovered from air in the exhaust flow path is utilized to reduce the energy required to condition the supply air.
Conventional energy exchange systems may utilize energy recovery devices (for example, energy wheels and permeable plate exchangers) or heat exchange devices (for example, heat wheels, plate exchangers, heat-pipe exchangers and run-around heat exchangers) positioned in both the supply air flow path and the exhaust air flow path. Liquid-to-Air Membrane Energy Exchangers (LAMEEs) are fluidly coupled so that a desiccant liquid flows between the LAMEEs in a run-around loop, similar to run-around heat exchangers that typically use aqueous glycol as a coupling fluid. When the only auxiliary energy used for such a loop is for desiccant liquid circulation pumps and external air-flow fans, the run-around system is referred to as a passive run-around membrane energy exchange (RAMEE) system, otherwise it is an active RAMEE system with controlled auxiliary heat and/or water inputs or extractions.
For the passive RAMEE system with one or more LAMEEs in each of the exhaust and supply air ducts, energy in the form of heat and water vapor is transferred between the LAMEEs in the supply and exhaust ducts, which is interpreted as the transfer of sensible (heat) and latent (moisture) energy between the exhaust air and the supply air. For example, the exhaust air LAMEE may recover heat and moisture from the exhaust air to transfer the heat and moisture to the supply air during winter conditions to heat and humidify the supply air. Conversely, during summer conditions, the supply air LAMEE may transfer heat and moisture from the supply air to the exhaust air to cool and dehumidify the supply air.
A Dedicated Outdoor Air System (DOAS) is an example of an HVAC system that typically does not return conditioned air back to the supply stream, but typically conditions ambient air to desired supply air conditions through a combination of heating, cooling, dehumidification, and/or humidification. A typical DOAS may include a vapor compression system or a liquid desiccant system. When the ambient air is hot and humid, the vapor compression system cools the supply air down to its dewpoint in order to dehumidify the air, which typically overcools the air. This process is inefficient because the air typically is reheated before it is supplied.
On the other hand, a liquid desiccant system does not overcool the supply air. However, traditional liquid desiccant systems typically require significantly more energy to condition the air. Moreover, a liquid desiccant system is generally a direct contact system, which is susceptible to transporting aerosolized desiccant downstream, where it may damage HVAC equipment.